Based on sound not counting my own language
Italian. From my own experiences dipping into the other four and comparing it to my other native language:
Spanish - like said above 9/10 hispanophones do not pronounce the z as thhh. Only double letters are rr and ll and both have their own sound and it's not like Italian where you must pronounce each consonant (example below). Grammar is pretty straightforward, biggest complaint I hear tutoring people is trouble with rr.
French: Harder grammar than Spanish, but easier than Romanian; biggest challenge is the spelling...many silent letters and rules. Teaching people about nasalization is also a biggie.
Italian: Very phonetic, but each double consonant means double the sound: bella = "bel-la" not "bela". For native speakers of languages that don't do this (like me) this can be a challenge. More declensions on nouns than Spanish too. Contrary to popular belief it it not as easy to understand if you speak Spanish without some basic learning. I understood some but to understand more than half studying it a bit REALLY helps.
Portuguese: Written a lot can be understood by Spanish speakers, but spoken is harder because it's very phonetic but has its little rules. Grammarwise on par with Spanish. There is also some degree of nasalization like French.
Romanian: One word: declensions. One noun can have many endings, this a trait of Latin that the language kept more closely than the others. Another is the vowel î which is a sound not present in the other four. This is the language I understand least from the Romance group,
French I can understand more in
spelling.
I speak Spanish and can make out a lot of Portuguese if it's academic based.
While some Spanish speakers/accent kind of "ruin" the language by over-rolling their "R"s or speaking with a "lisp" (Seville 
), it is such a beautiful language both written and spoken. French is overrated IMO. The French language written is cute, but Spanish is too. It sounds really fancy/elegant when it's spoken, but I personally don't think it should be considered the main language of love. I haven't heard enough Italian to give a legitimate opinion on it.